Method and device for producing formic acid formates and use of said formates

ABSTRACT

The invention relates to a method for producing formic acid formates, whereby (a) formic acid methyl ester is partially hydrolysed with water; (b) formic acid methyl ester and methanol are separated by distillation from the reaction mixture obtained in step (a), forming a current containing formic acid and water; (c) the current obtained in step (b), containing the formic acid methyl ester and optionally methanol, is converted into a current containing formate and water, by (i) reaction with a basic compound having a pK a  value of the corresponding acid of the corresponding dissociation step of =3, measured at 25° C. in an aqueous solution, in presence of water, and (ii) separation by distillation of the methanol; and (d) the current obtained in step (b), containing formic acid and water, and the current obtained in step (c), containing formate and water, are combined to form a mixture containing the formic acid formate and water. The invention also relates to a device for producing said formic acid formates and to the use of the same.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a National Stage application ofPCT/EP2003/008399, filed Jul. 30, 2003, which claims priority fromGerman Patent Application No. DE 102 37 379.5, filed Aug. 12, 2002.

In addition, the invention relates to the use of the acid formates forpreserving and/or acidifying plant and/or animal materials, for treatingbiowastes and as an additive in animal nutrition and/or as growthpromoters for animals.

Acid formates have an antimicrobial activity and are used, for example,for preserving and for acidifying plant and animal materials, forinstance grasses, agricultural products or meat, for treating biowastesor as an additive for animal nutrition.

Acid formates and preparation methods for these have long been known.Thus, Gmelins Handbuch der anorganischen Chemie [Gmelin's Handbook ofInorganic Chemistry], 8^(th) edition, Number 21, pages 816 to 819,Verlag Chemie GmbH, Berlin 1928 and Number 22, pages 919 to 921, VerlagChemie GmbH, Berlin 1937 describes the synthesis of sodium diformate andof potassium diformate by dissolving sodium formate and potassiumformate in formic acid. The crystalline diformates may be obtained bydecreasing the temperature and by evaporating off excess formic acid.

DE 424 017 teaches preparing acid sodium formates having varying acidcontent by introducing sodium formate into aqueous formic acid in anappropriate molar ratio. By cooling the solution the correspondingcrystals can be obtained.

According to J. Kendall et al., Journal of the American ChemicalSociety, Vol. 43, 1921, pages 1470 to 1481, acid potassium formates maybe obtained by dissolving potassium carbonate in 90% strength formicacid, forming carbon dioxide. The corresponding solids can be obtainedby crystallization.

U.S. Pat. No. 4,261,755 describes preparing acid formates by reacting anexcess of formic acid with the hydroxide, carbonate or bicarbonate ofthe corresponding cation.

WO 96/35657 teaches preparing products which contain disalts of formicacid by mixing potassium formate, hydroxide, carbonate or bicarbonate,sodium formate, hydroxide, carbonate or bicarbonate, cesium formate,hydroxide, carbonate or bicarbonate or ammonium formate or ammonia with,possibly aqueous, formic acid, subsequently cooling the reactionmixture, filtering the resultant slurry and drying the resultant filtercake and recirculating the filtrate.

A disadvantage of the abovementioned processes is that, per mole offormate formed by the reaction with the basic compounds, in each caseone mole of formic acid is consumed. This is because, as is known, it isprecisely the preparation of concentrated, that is to say substantiallyanhydrous, formic acid, which is a process which requires extensiveequipment, and is costly and energy-consuming. Thus the abovementionedprocesses, based on the entire value-added chain, require extensiveequipment and are costly and energy-consuming.

German application No. 102 10 730.0 teaches preparing acid formates byreacting methyl formate with water and a basic compound having a pK_(a)of the conjugate acid of the appropriate dissociation state of ≧3, andsubsequently removing the methanol formed and optionally setting thedesired acid content by adding formic acid.

German application No. 101 54 757.9 teaches preparing metalformate/formic acid mixtures by carbonylating the corresponding metalhydroxide to give the metal formate in the presence of water and acatalyst, removing the water and the catalyst by distillation and addingformic acid to the metal formate to produce the desired metalformate/formic acid mixture.

It is an object of the present invention, therefore, to provide aprocess which no longer has the abovementioned disadvantages, whichmakes it possible to prepare acid formates on an industrial scale inhigh yield and high space-time yield, with simultaneously highflexibility with respect to composition and with the use of readilyaccessible raw materials and which permits a simple process procedurewith low capital costs and low energy consumption.

We have found that this object is achieved by a process for preparingacid formates, which comprises

-   (a) partially hydrolyzing methyl formates with water;-   (b) separating off by distillation methyl formate and methanol from    the reaction mixture obtained in process stage (a), forming a stream    comprising formic acid and water;-   (c) converting the stream comprising methyl formate with or without    methanol from the process stage (b) by    -   (i) reaction with a basic compound having a pK_(a) of the        conjugate acid of the appropriate dissociation state of ≧3,        measured at 25° C. in aqueous solution, in the presence of        water, and    -   (ii) removal of the methanol by distillation, into a stream        comprising formate and water; and-   (d) combining the stream comprising formic acid and water from the    process stage (b) and the stream comprising formate and water from    the process stage (c), forming a mixture comprising the acid formate    and water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a representative schematic of a process flow chartillustrating a first preferred embodiment of the invention; and

FIG. 2 shows a representative schematic of a process flow chartillustrating a second preferred embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Acid formates are compounds and mixtures which contain formate anions(HCOO⁻), cations (M^(x+)) and formic acid (HCOOH). They can occurtogether in the form of a solid or a liquid and if appropriate containother components, for example other salts, additives or solvents such aswater. Generally, the acid formates can be represented by the formulaHCOO⁻M^(x+) _(1/x)*y HCOOH  (I),where M is a monovalent or polyvalent inorganic or organic cation, x isa positive integer and indicates the charge of the cation and y givesthe molar fraction of formic acid based on the formate anion. The molarfraction of formic acid based on the formate anion y is generally from0.01 to 100, preferably from 0.05 to 20, particularly preferably from0.5 to 5, and in particular from 0.9 to 3.1.

The nature of the inorganic or organic cation M^(x+) is in principle notcritical, provided that this is stable under conditions under which theacid formate is to be handled. This also includes, for example,stability toward the reducing formate anion. Possible inorganic cationsare the monovalent and/or polyvalent metal cations of metals of groups 1to 14 of the Periodic Table of the Elements, for example lithium (Li⁺),sodium (Na⁺), potassium (K⁺), cesium (Cs⁺), magnesium (Mg²⁺), calcium(Ca²⁺), strontium (Sr²⁺) and barium (Ba²⁺), preferably sodium (Na⁺),potassium (K⁺), cesium (Cs⁺) and calcium (Ca²⁺). Possible organiccations are unsubstituted ammonium (NH₄+) and ammonium substituted byone or more carbon-containing radicals which may also be linked to oneanother, for example methylammonium, dimethylammonium,trimethylammonium, ethylammonium, diethylammonium, triethylammonium,pyrollidinium, N-methylpyrroldinium, piperidinium, N-methylpiperidiniumor pyridinium.

A carbon-containing organic radical is an unsubstituted or substituted,aliphatic, aromatic or araliphatic radical having from 1 to 30 carbons.This radical can contain one or more heteroatoms, such as oxygen,nitrogen, sulfur or phosphorus, for example —O—, —S—, —NR—, —CO—, —N═,—PR— and/or —PR₂ and/or be substituted by one or more functional groupswhich, for example, contain oxygen, nitrogen, sulfur and/or halogen, forexample by fluorine, chlorine, bromine, iodine and/or a cyano group (theradical R here is also a carbon-containing organic radical). Thecarbon-containing organic radical can be a monovalent or polyvalentradical, for example divalent or trivalent radical.

The individual process stages are described in more detail below:

Process Stage (a)

In process stage (a), methyl formate is partially hydrolyzed with waterto formic acid and methanol. Partially means that only a portion of themethyl formate fed is hydrolyzed.

In the inventive process, in process stage (a) processes which are knownper se for hydrolyzing methyl formate can be used. A general overview ofknown and industrially relevant processes for hydrolysis is given, forexample, in Ullmann's Encyclopedia of Industrial Chemistry, 6^(th)edition, 2000 electronic release, Chapter “FORMIC ACID, Production”.Other suitable hydrolysis processes are also described, for example, inEP-A 0 005 998 and EP-A 0 017 866.

The hydrolysis is generally carried out at a temperature of from 80 to150° C. and a pressure of from 0.5 to 2.0 MPa absolute. Reactionapparatuses which can be used are in principle all reaction apparatuseswhich are suitable for reactions in the liquid phase. Examples arestirred tanks and jet loop reactors. Preference is given to the use of acascade reactor.

Generally, it is advantageous to carry out the hydrolysis in thepresence of an acid catalyst, since this significantly increases thehydrolysis rate. Acid catalysts which can be used are the formic acidwhich is formed or additional catalysts. The additional catalysts can beof homogeneous or heterogeneous nature. Examples of heterogeneouscatalysts are acid ion-exchangers, for example polysulfonic acids orpoly(perfluoroalkylene)sulfonic acids (for example Nafion® from Du Pont)and examples of homogeneous catalysts are strong inorganic or organicacids, such as sulfuric acid, hydrochloric acid or alkyl- andtolylsulfonic acids. If homogeneous catalysts are used, these mustgenerally be removed in a subsequent stage. Depending on the desiredpurity of the acid formates to be prepared, however, it is also possibleto allow these to remain in the system. In this case, the acid catalystsare usually recovered in the form of their salts in the acid formate.Particularly preferably, the partial hydrolysis is carried out in thepresence of formic acid as acid catalyst, which avoids adding anadditional catalyst and its subsequent removal or possible contaminationof the acid formates. Generally, for this purpose, at the reactor inleta formic acid concentration of from about 0.1 to 2% by weight, based onthe liquid mixture present which contains water and methyl formate, isestablished, by targeted addition of formic acid or a stream comprisingformic acid.

The molar ratio of methyl formate to water to be used in the hydrolysisin the inventive process is generally from 0.1 to 10. Since this is anequilibrium reaction, an excess of water is preferably used, as alsofollows, for example, from the teaching of EP-A 0 017 866. Preferably inprocess stage (a), the methyl formate and the water are fed in a molarratio of from 0.1 to 1, and particularly preferably from 0.15 to 0.3.

The reaction mixture obtained from the partial hydrolysis thus comprisesunreacted methyl formate, formic acid, methanol and, owing to thepreferred use of an excess of water, water.

Preferably, the aqueous reaction mixture comprises from 5 to 15 mol %,particularly preferably from 8 to 12 mol %, formic acid, from 3 to 10mol %, particularly preferably from 6 to 12 mol %, methyl formate andfrom 6 to 15 mol %, particularly preferably from 8 to 12 mol %,methanol.

Process Stage (b)

In process stage (b), methyl formate and methanol are removed bydistillation from the reaction mixture obtained in process stage (a),forming a stream comprising formic acid and water. Methyl formate andmethanol can here in principle be removed together in the form of astream or separately in the form of a stream comprising methyl formateand a stream comprising methanol. Generally, methyl formate and methanolare taken off separately or together in the upper part of the column.The stream comprising formic acid and water is generally taken off fromthe bottom. Preference is given in process stage (b) to the jointremoval of a stream comprising methyl formate and methanol.

The design and operation of the distillation column is primarilydependent on the composition of the stream which is fed and on thedesired purities of the two product streams and can be determined in aknown way by those skilled in the art.

Process Stage (c)

In process stage (c), the stream comprising methyl formate with orwithout methanol from the process stage (b) is converted by

-   (i) reaction with a basic compound having a pK_(a) of the conjugate    acid of the appropriate dissociation state of ≧3, measured at 25° C.    in aqueous solution, in the presence of water, and-   (ii) removal of the methanol by distillation    into a stream comprising formate and water.

The basic compound to be used preferably has a pK_(a) of the conjugateacid of the appropriate dissociation state of 23.5, particularlypreferably ≧9, and very particularly preferably ≧10, measured at 25° C.in aqueous solution. The basic compound can be of inorganic or organicnature. The basic compound can be a salt or a covalent compound. Theconjugate acid of the appropriate dissociation state is the acid formedby formal addition of a proton (H⁺).

In the event that the basic compound is a salt, this can generally berepresented by the formulaM^(x+) _(a)A^(a−) _(x)  (II),where M and x have the meaning specified under (I) and A is an inorganicor organic anion having the charge “a−”. The conjugate acid of theappropriate dissociation state is thus HA^((a−1)−). The appropriatedissociation equation defining the pK_(a) to be considered is as followsHA^((a−1)−)⇄A^(a−+)H⁺  (III).

In the event that the basic compound is a covalent compound B, thedissociation equation defining the pK_(a) to be used is as followsHB⁺⇄B+H⁺  (IV).

Examples of suitable basic compounds are the salts M^(x+) _(a)A^(a−)_(x) (II), where M^(x+) is a monovalent or polyvalent metal cation of ametal as described above and A^(a−) is an anion as listed in Table 1aand also the covalent compounds B as listed in Table 1b.

TABLE 1a Possible anions A^(a−) of suitable basic compounds and pK_(a)s(measured at 25° C. in aqueous solution) of the conjugate acids of theappropriate dissociation states. Anions A^(a−) Conjugate acid pK_(a)Hydroxide (OH⁻) Water (H₂O) 14.0 Carbonate (CO₃ ²⁻) Hydrogen carbonate(HCO₃ ⁻) 10.3 Hydrogen carbonate Carbonic acid (H₂CO₃) 6.4 (HCO₃ ⁻)Borate (BO₃ ³⁻) Hydrogen borate (HBO₃ ²⁻) >14 Hydrogenborate (HBO₃ ²⁻)Dihydrogen borate (H₂BO₃ ⁻) >14 Dihydrogenborate (H₂BO₃ ⁻) Boric acid(H₃BO₃) 9.3 Phosphate (PO₄ ³⁻) Hydrogen phosphate (HPO₄ ²⁻) 12.3Hydrogenphosphate Dihydrogen phosphate 7.2 (HPO₄ ²⁻) (H₂PO₄ ⁻) FormateFormic acid 3.8 Acetate Acetic acid 4.8 Propionate Propionic acid 4.9Oxalate (C₂O₄ ²⁻) Hydrogen oxalate (HC₂O₄ ⁻) 4.2 2-Ethylhexanoate2-Ethylhexanoic acid >4 (C₄H₉—CH(C₂H₅)—COO⁻) (C₄H₉—CH(C₂H₅)—COOH) >4

TABLE 1b Possible covalent bases B as suitable basic compounds andpK_(a)s (measured at 25° C. in aqueous solution) of the conjugate acidsof the appropriate dissociation states. Covalent base B Conjugate acidpK_(a) Ammonia Ammonium 9.3 Methylamine Methylammonium 10.6Dimethylamine Dimethylammonium 10.7 Trimethylamine Trimethylammonium 9.8Ethylamine Ethylammonium 10.7 Diethylamine Diethylammonium 11.0Triethylamine Triethylammonium 10.8 Pyrollidine Pyrollidinium 11.3N-Methylpyrrolidine N-Methylpyrroldinium 10.3 Piperidine Piperidinium11.1 N-Methylpiperidine N-Methylpiperidinium 10.1 Pyridine Pyridinium5.3

Preferably, in the inventive process, the basic compound used is lithiumhydroxide, lithium hydrogen carbonate, lithium carbonate, sodiumhydroxide, sodium hydrogen carbonate, sodium carbonate, potassiumhydroxide, potassium hydrogen carbonate, potassium carbonate, ammoniumcarbonate, ammonium hydrogen carbonate and/or ammonia, particularlypreferably sodium hydroxide, sodium hydrogen carbonate, sodiumcarbonate, potassium hydroxide, potassium hydrogen carbonate, potassiumcarbonate and/or ammonia, and particularly preferably sodium hydroxide,sodium carbonate, potassium hydroxide and/or potassium carbonate, inparticular sodium hydroxide and/or potassium hydroxide.

The manner in which the basic compounds are added is generally notimportant in the inventive process. They can be added in solid, liquidor gaseous form, as pure substance, as a mixture of substances or assolution. Examples are the addition in the form of aqueous solutions(for example aqueous solutions of the alkali metal salts or ammoniawater), in the form of solid compounds (for example powders of thealkali metal salts), in the gaseous state (for example gaseousammnonia). Preference is given to addition in the form of their aqueoussolutions.

The order in which the starting materials are added is also generallynot important in the inventive process. Thus it is possible, forexample, to charge the basic compound in solid or liquid form (forexample as aqueous solution) and then to introduce the stream comprisingmethyl formate in liquid or gaseous form. It is, in addition, possibleto charge in liquid form the stream comprising methyl formate and thento add the basic compound. In addition, it is obviously also possibleand, in particular when a continuous process is being carried out,advantageous, to combine the stream comprising methyl formate and thebasic compound continuously.

The molar ratio of methyl formate to the basic compound is to be setadvantageously in the inventive process stoichiometrically, that is tosay in such a manner that the added methyl formate reacts with the addedbasic compound in accordance with the reaction stoichiometry to give thecorresponding formate and water. The critical parameter for this is whatis termed the molar equivalent of the basic compound, in which case hereall dissociation states which lead by addition of protons to conjugateacids which have a pK_(a) of ≧3, measured at 25° C. in aqueous solution,must be taken into account. Thus, when potassium hydroxide is used asbasic compound, preferably a molar ratio of methyl formate/potassiumhydroxide of 1.0 is to be chosen, since this corresponds to theformation of potassium formate:

When potassium carbonate is used as basic compound, preferably a molarratio of methyl formate/potassium carbonate of 2.0 is to be chosen,since the conjugate carbonic acid is dibasic:

Deviations above and below the above stoichiometric addition are alsopossible in the inventive process, however. Thus, in the event of adeficit of basic compound there is the risk of incomplete reaction ofthe methyl formate and thus the risk of contamination of the methanol,which is to be removed by distillation, with unreacted methyl formate.In the event of an excess of basic compound, the resultant stream would,in addition to the corresponding formate and water, still contain theresidual basic compound.

The amount of water to be used in the inventive process in the processstage (c) can vary over a broad range. Generally, from 20 to 90% byweight of water, and preferably from 40 to 50% by weight, based on theamount of methyl formate fed, is used in the reaction. Generally, thewater is added via an aqueous solution of the basic compound, althoughit is also possible to add pure water.

The stream comprising methyl formate is generally reacted in processstage (c) with said basic compound in the presence of water at atemperature of from 0 to 150° C., preferably from 30 to 120° C., andparticularly preferably from 50 to 80° C. During the procedure thepressure is generally from 0.05 to 1 MPa absolute, preferably from 0.08to 0.5 MPa absolute and particularly preferably from 0.09 to 0.15 MPaabsolute.

The reaction of the stream comprising methyl formate in process stage(c) with said basic compound in the presence of water is in principleindependent of the removal of methanol by distillation.

The removal of methanol by distillation can therefore, in the inventiveprocess, in principle take place before, during or after said reaction.Preferably, the methanol is removed by distillation together with, orafter, said reaction.

When the methanol is removed by distillation before or after saidreaction, in principle, all reaction apparatuses can be used for thereaction which are suitable for reactions in the liquid phase. Examplesare stirred tanks and jet loop reactors. The methanol is removed bydistillation here in a separate step, customarily in a distillationcolumn.

In the inventive process, particular preference is given to removing themethanol by distillation together with reacting the methyl formate withthe water and the basic compound, with conversion into the streamcomprising formate and water in one column. On account of the lowerboiling point of methyl formate compared with water, in this case thestream comprising methyl formate and methanol from the process stage (b)is advantageously added below the feed point of the water and the basiccompound. Since the methyl formate and the methanol ascend in the columnand the water and the basic compound flow downward, the column has aregion suitable for said reaction. The methanol ascends and can beisolated overhead. Since methyl formate is generally prepared bycarbonylating methanol, it is particularly advantageous to recirculatethe methanol isolated overhead as feed stock for the preparation ofmethyl formate, the recirculating methanol in this variant by all meansstill being able to comprise residual amounts of methyl formate. Thus itis merely necessary in the overall balance to replace the small methanollosses by fresh methanol.

The stream comprising the aqueous formate flows downward in the columnand is taken off as bottom stream. It can be advantageous here towithdraw a portion of the water as side stream at the bottom end of thecolumn and to recirculate it to the hydrolysis. As a result of thismeasure, a more highly concentrated aqueous solution of thecorresponding formate is also achieved.

The necessary residence time in the saponification part of the columncan be provided, for example, by suitable internals, for exampleThormann plates, or if appropriate by an external reaction volume. Whenan external reaction volume is provided, the stream to be saponified iswithdrawn from the column at a suitable point via a side stream takeoff,fed to the external reaction apparatus and fed back to the column at asuitable point. In the context of the present invention, both variantsare considered primarily equivalent.

The column is designed in a manner known and customary to those skilledin the art.

Process Stage (d)

In process stage (d), the stream comprising the formic acid and thewater from process stage (b) and the stream comprising formate and waterfrom process stage (c) are combined, forming a mixture comprising theacid formate and water.

The sequence of addition of the stream containing formic acid and thewater from process stage (b) and the stream comprising the formate andwater from process stage (c) is in general not critical in the inventiveprocess. In particular, it is possible, and may be advantageous, tosubject the stream comprising the formic acid and the water from processstage (b) and/or the stream comprising the formate and water from theprocess stage (c), before they are combined, to a concentration informic acid or formate. In particular, the removal of a portion of thewater present by evaporation, preferably by distillation, may bementioned for this.

Temperature and pressure for the combining in process stage (d) aregenerally not critical. Generally, they are combined at a temperature offrom 0 to 150° C. and a pressure of from 0.01 to 0.3 MPa absolute.

The apparatuses used can in principle be all apparatuses which aresuitable for reactions in the liquid phase and, if appropriate, forreactions in the liquid phase with simultaneous removal of a volatilecomponent. Examples are stirred tanks, jet loop reactors and columns. Inaddition, it is also possible to combine the two streams by theirmeeting within a pipe, advantageously having a downstream mixingsection. In addition, it is also possible to combine the two streams inthe apparatus in which solid acid formate is isolated.

The mixture obtained by combining the stream comprising the formic acidand the water from the process stage (b) and the stream comprising theformate and water from process stage (c) comprises the acid formate inthe form of an aqueous solution, with or without previously precipitatedacid formate as solid. Depending on requirements, in this form, it canbe packaged, stored, transported and/or used for appropriateformulations or uses. In addition, the acid formate can be furtherconcentrated or isolated as solid by downstream process steps.

Preference is given to a variant in which, in process stage (d),

-   (i) the stream comprising the formic acid and the water from the    process stage (b), together with the mother liquor recirculated from    step (iv), is concentrated in a column or an evaporator with removal    of water by distillation;-   (ii) the stream which was produced from step (i) by concentration    and comprises formic acid, water and formate is combined with the    stream comprising the formate and water from the process stage (c)    forming a mixture comprising the acid formate and water;-   (iii) solid acid formate from the mixture comprising acid formate    and water obtained from step (ii) is precipitated by crystallization    and this is isolated; and-   (iv) the resultant mother liquor is recirculated to step (i).

The column or the evaporator in step (i) is generally to be operated insuch a manner that a portion of the water fed can be taken off, forexample overhead. The remaining stream comprising formic acid, water andformate generally has a water content of from 10 to 40% by weight and iswithdrawn as bottom product. Said procedure has the advantage of acertain concentration of the stream comprising the formic acid and theformate. The water withdrawn from the column or the evaporator isadvantageously recirculated to the hydrolysis stage in process step (a)and the excess is taken off from the process. The column or evaporatoris designed in a manner known and customary to those skilled in the art.

The stream which is produced by concentration and comprises formic acid,water and formate can be combined with the stream comprising formate andwater from process stage (c) forming a mixture comprising the acidformate and water in step (ii), for example, between the column and thecrystallization apparatus, for example by combining two lines, or theycan be combined in a separate mixing apparatus, or in thecrystallization apparatus itself.

The crystallization procedure is generally known to those skilled in theart, with the precise design and procedure being able to take place inthe customary manner. Generally, the crystallization is carried out at atemperature in the range from −20° C. to +80° C., and preferably from 0°C. to 60° C. Generally, the amount of product crystallized out increaseswith decreasing temperature. The crystallization can in principle becarried out in all known apparatuses for this. Said embodiment isparticularly advantageously usable for removing acid formates which cancrystallize in the desired composition. Relevant examples are potassiumdiformate (HCOOK.HCOOH), sodium diformate (HCOONa.HCOOH), sodiumtetraformate (HCOONa.3 HCOOH) or mixtures thereof. The formates or acidformates which are crystallized out are generally removed by customaryand known methods, for example by filtration or centrifugation.

The mother liquor which is produced in the crystallization of the solidacid formate is recirculated in step (iv) to step (i). Since this stillcomprises a considerable proportion of product of value, this thus alsoensures its isolation. However, alternatively, it is also possible touse the value present in the mother liquor in a different manner, forexample by direct use as solution.

Likewise, preference is given to a variant in which, in process stage(d)

-   (i) the stream from the process stage (b) comprising the formic acid    and the water and the stream from the process stage (c) comprising    the formate and the water are combined to form a mixture comprising    the acid formate and water in a column or an evaporator with removal    of water by distillation; and-   (ii) solid acid formate is separated off by spray granulation, spray    drying or melt crystallization from the mixture obtained from    step (i) comprising acid formate and water, and this solid acid    formate is isolated.

The two streams can be combined in step (i) upstream of the column orthe evaporator, for example by joining two lines, or they can becombined in a separate mixing apparatus, or in the column or theevaporator, for example via two separate feeds.

The column or the evaporator in step (i) is generally to be operated insuch a manner that a portion of the water fed can be taken off, forexample overhead. The remaining acid-formate-containing mixture, whichgenerally has a water content of from 0.5 to 30% by weight, is withdrawnas bottoms product. In particular in the isolation of the acid formateby means of melt crystallization, a lower water content generally from<1% by weight is set in the bottoms product. Said procedure has theadvantage of a certain concentration of the stream comprising the acidformate. The water withdrawn from the column or the evaporator isadvantageously recirculated to the hydrolysis stage in process step (a)and the excess taken off from the process. The column or the evaporatoris designed in the manner known and customary to those skilled in theart.

The spray granulation, spray drying and melt crystallization proceduresare generally known to those skilled in the art, in which case theprecise design and procedure can be carried out in the customary manner.The abovementioned methods can also particularly advantageously be usedfor removing acid formates which can be crystallized in the desiredcomposition. Relevant examples are potassium diformate (HCOOK*HCOOH),sodium diformate (HCOONa*HCOOH), sodium tetraformate (HCOONa*3 HCOOH) ormixtures thereof.

Since in the spray granulation, the spray drying and the meltcrystallization, advantageously an aqueous acid formate having a lowwater content can be used, generally, also, only a small proportion ofcondensate or free amino acid is obtained.

Depending on the amount produced and the residual concentration of acidformate present, it may also be advantageous not to recirculate thestream, but to eject it from the system.

The inventive process can be carried out in principle batchwise,semicontinuously or continuously. Preferably, the inventive process iscarried out continuously.

Preferably, in the inventive process, the acid formate prepared is acidmetal formates, particularly preferably acid potassium formate, acidsodium formate, acid calcium formate or mixtures thereof and veryparticularly preferably potassium diformate (HCOOK.HCOOH), sodiumdiformate (HCOONa.HCOOH), sodium tetraformate (HCOONa.3 HCOOH) ormixtures thereof.

The acid formates are generally prepared in the form of their solutions,or crystalline as solids. If appropriate they can further be admixedwith other components, for example other formate salts. In the case ofthe crystalline acid formates, it is generally advantageous for storage,transport and use, to compact these together with a dessicant, forexample silicates or starch, to form a particulate compactate or diverseshaped bodies, for example tablets or beads.

In a particularly preferred embodiment, the simplified process flowchart which is shown in FIG. 1, via line (1), methyl formate and watercomprising formic acid which is recirculated from the process are addedto the cascade hydrolysis reactor (A). Generally, the two startingmaterials are brought to the desired inlet temperature in a heatexchanger premixed (as shown in the flow chart) or separately. Thereaction mixture originating from the hydrolysis stage (process stage(a)), which reaction mixture comprises unreacted methyl formate, water,formic acid and methanol, is fed via line (2) to the column (B) in whichthe reaction mixture is separated by distillation into an overheadstream comprising methyl formate and methanol, and a bottoms streamcomprising aqueous formic acid (process stage (b)). The overhead streamcomprising methyl formate and methanol is fed via line (3) to column(C). In addition, the aqueous basic compound, particularly preferablypotassium hydroxide solution, is fed to the column (C) above the inletpoint of the stream comprising methyl formate and methanol via line (5).Methanol is recovered overhead from column (C) and is preferablyrecirculated for repeated preparation of methyl formate bycarbonylation. At the bottom end of column (C), a portion of the wateris withdrawn and recirculated via line (6) to the hydrolysis stage. Thebottoms product obtained is an aqueous potassium formate solution. Thestream comprising aqueous formic acid from process stage (b) is fed vialine (7) to the column (D). If appropriate, a portion of the streamcomprising the aqueous formate solution from process stage (c) is alsofed via lines (8) and (8 b). The column (D) is advantageously operatedin such a manner that the bottoms product obtained is a concentratedmixture comprising formic acid, formate and water having a water contentof generally from 10 to 40% by weight. A portion of the water iswithdrawn from the column (D) in the form of a formic-acid-containingwater stream as overhead product and recirculated via line (13) to thehydrolysis stage. A portion of the water stream comprising small amountsof formic acid can here optionally be withdrawn from the system via line(12). The bottoms product of column (D) is fed via line (9) to anapparatus (E) suitable for crystallization, for example a cooling disccrystallizer. The stream comprising the aqueous formate solution is fedfrom the process stage (C) via line (8 a). The feed in this case can beperformed, for example, by combining two lines (as shown in FIG. 1) ordirectly into the crystallization apparatus. The crystallization isprimarily performed by temperature decrease. The resultant crystals arefed together with the supernatant solution for separation to theapparatus (F). Preferably the separation is performed by centrifugation.The separated crystals are withdrawn via line (10) and can be driedand/or compounded, for example in optional following stages. Theresultant mother liquor is recirculated via line (11) to the column (D).

In another particularly preferred embodiment, whose simplified processflow chart is shown in FIG. 2, the process stages (a), (b) and (c) arecarried out as described in the above particularly preferred embodiment.The stream comprising the aqueous formic acid from the process stage (b)is fed via line (7) and the stream comprising the aqueous formatesolution from the process stage (c) is fed via line (8) to the column(D). The column (D) is advantageously operated in such a manner that thebottom product obtained is a concentrated mixture comprising formicacid, formate and water having a water content generally from 0.5 to 30%by weight. A portion of the water fed is withdrawn from the column (D)as overhead product in the form of a formic-acid-containing water streamand is recirculated via line (13) to the hydrolysis stage. A portion ofthe water stream comprising small amounts of formic acid can hereoptionally be withdrawn from the system via line (12). The bottomsproduct of the column (D) is fed via line (9) to an apparatus (E)suitable for spray granulation, spray drying or melt crystallization.The resultant solid acid formate is withdrawn via line (10) and can bedried and/or compounded, for example in optional following stages. Theresultant condensate can optionally be recirculated via line (11) to thecolumn (D) or ejected from the system.

The inventive process makes it possible to prepare acid formates on anindustrial scale in high yield and high space-time yield, withsimultaneously high flexibility with respect to composition and usingreadily accessible raw materials with simple process design and lowcapital costs. In addition, the process has the critical advantage thatnot only the formate, but also the formic acid, can be produced directlyfrom the methyl formate without the costly diversion, which is expensivein terms of resources, via the concentrated formic acid. The inventiveprocess is therefore simple to carry out in processing terms andcompared with the processes involving direct use of concentrated formicacid according to the prior art, has markedly lower capital costs and amarkedly lower energy consumption. In addition, in part the use ofhigh-alloy steels can be avoided, since the acid formates are much lesscorrosive than concentrated formic acid.

In addition, the invention relates to an apparatus for preparing theacid formates according to the inventive process, comprising

-   (a) a reactor (A) suitable for hydrolyzing methyl formate;-   (b) a column (B) suitable for separating by distillation a stream    comprising methyl formate, formic acid, methanol and water into    methyl formate, methanol and a stream comprising formic acid and    water, which column is connected on the feed side to the reactor    (A);-   (c) a column (C) suitable for saponifying methyl formate with a    basic compound and for removing methanol by distillation, which    column is connected on the feed side to the column top of column (B)    and has above said feed an inlet point for the basic compound; and-   (d) a column (D) suitable for removing water from a stream    comprising formic acid and water, which column is connected on the    feed side to the column bottom of column (B).

A suitable reactor (A) is, for example, a stirred tank or a jet loopreactor. Preference is given to a cascade reactor. The reactor (A) isdesigned according to the manner customary and known to those skilled inthe art.

The column (B) is designed in the manner which is customary and known tothose skilled in the art.

The column (C) can comprise suitable internals in the saponificationpart for providing the residence time required for the process, forinstance Thormann plates, or if appropriate an external reaction volumeconnected to the column. The external reaction volume which may bepresent is generally connected to the column via a suitable side streamtakeoff and a suitable side stream feed. The column (C) is designed inthe manner which is customary and known to those skilled in the art.

The column (D) is designed in the manner customary and known to thoseskilled in the art.

A preferred apparatus is an apparatus which, in addition to theabovementioned features (a) to (d), comprises

-   (e) an apparatus (E) suitable for crystallizing acid formate, which    apparatus is connected on the feed side to the column bottom of    column (D) and to the column bottom of column (C);-   (f) an apparatus (F) suitable for separating off crystals of the    acid formate, which apparatus is connected on the feed side to    apparatus (E); and-   (g) a connection line (11) between apparatus (F) and column (D),    which connection line is suitable for recirculating mother liquor.

The apparatuses (E) and (F) are designed in the manner which iscustomary and known to those skilled in the art.

Furthermore, the preferred apparatus is an apparatus which, in additionto the abovementioned features (a) to (d), comprises

-   (e) a connection line (8) between the column bottom of column (C)    and column (D), which connection line is suitable for feeding    aqueous formate; and-   (f) an apparatus (E) suitable for spray granulation, spray drying or    melt crystallization, which apparatus is connected on the feed side    to the column bottom of column (D).

The apparatus (E) is designed in the manner which is customary and knownto those skilled in the art.

In addition, the invention relates to the use of the inventivelyprepared acid formates for preserving and/or acidifying plant and animalmaterials. Examples are the use of acid formates for preserving andacidifying grass, agricultural plants, fish and fish products and meatproducts, as are described, for example, in WO 97/05783, WO 99/12435, WO00/08929 and WO 01/19207.

Furthermore, the invention relates to the use of the inventivelyprepared acid formates for treating biowastes. The use of acid formatesfor treating biowastes is described, for example, in WO 98/20911.

In addition, the invention relates to the use of the inventivelyprepared acid formates as additive in animal nutrition and/or as growthpromoters for animals, for example for breeding sows, growing/finishingpigs, poultry, calves, cows and fish. Said use is described, forexample, in WO 96/35337. Preference is given to the use of theinventively prepared acid potassium formates, in particular potassiumdiformate, as additive in animal nutrition and/or as growth promotersfor animals, in particular for breeding sows and growing/finishing pigs.

Very particularly preferred mixtures for the preferred use of the acidpotassium formates prepared by the inventive process as additive inanimal nutrition and/or as growth promoters for animals are thefollowing two compositions:

Mixture 1 Mixture 2 (% by (% by weight) weight) Potassium diformate 20to 60 60 to 99 Sodium diformate/tetraformate 20 to 50 — Calcium formate 0 to 25  0 to 28 Dessicant (silicate or starch) 0 to 4 0 to 4 Water 0to 5 0 to 5

Very particular preference is given to the use of the inventivelyprepared potassium diformate as additive in animal nutrition and/or asgrowth promoter for animals in the form of a product of composition98.0±1% by weight potassium diformate, 1.5±1% by weight silicate and0.5±0.3% by weight water.

1. A process for preparing acid formates comprising: (a) partiallyhydrolyzing methyl formates with water; (b) separating off bydistillation methyl formate and methanol from the reaction mixtureobtained in process stage (a), forming a stream comprising formic acidand water; (c) converting the stream comprising methyl formate with orwithout methanol from the process stage (b) by (i) reaction with a basiccompound having a pK_(a) of the conjugate acid of the appropriatedissociation state of ≧3, measured at 25° C. in aqueous solution, in thepresence of water, and (ii) removal of the methanol by distillation,into a stream comprising formate and water; and (d) combining the streamcomprising formic acid and water from the process stage (b) and thestream comprising formate and water from the process stage (c), forminga mixture comprising the acid formate and water.
 2. The processaccording to claim 1, wherein, in the process stage (a), the methylformate and the water are fed in a molar ratio of 0.1 to
 1. 3. Theprocess according to claim 1, wherein, in the process stage (c), theremoval of the methanol by distillation and the reaction of the methylformate with the water and basic compound with transfer into the streamcomprising formate and water are carried out together in one column. 4.The process according to claim 1, wherein, in the process stage (d): (i)the stream comprising the formic acid and the water from the processstage (b), together with the mother liquor recirculated from step (iv),is concentrated in a column or an evaporator with removal of water bydistillation; (ii) the stream which was produced from step (i) byconcentration and comprises formic acid, water and formate is combinedwith the stream comprising the formate and water from the process stage(c) forming a mixture comprising the acid formate and water; (iii) solidacid formate from the mixture comprising acid formate and water obtainedfrom step (ii) is precipitated by crystallization and this is isolated;and (iv) the resultant mother liquor is recirculated to step (i).
 5. Theprocess according to claim 1, wherein, in process stage (d): (i) thestream from the process stage (b) comprising the formic acid and thewater and the stream from the process stage (c) comprising the formateand the water are combined to form a mixture comprising the acid formateand water in a column or an evaporator with removal of water bydistillation; and (ii) solid acid formate is separated off by spraygranulation, spray drying or melt crystallization from the mixtureobtained from step (i) comprising acid formate and water, and this solidacid formate is isolated.
 6. The process according to claim 1, wherein,in process step (c), the basic compound is selected from the groupconsisting of sodium hydroxide, sodium hydrogen carbonate, sodiumcarbonate, potassium hydroxide, potassium hydrogen carbonate, potassiumcarbonate and ammonia.
 7. The process according to claim 1, wherein theacid formate prepared is selected from the group consisting of acidpotassium formate, acid sodium formate, acid calcium formate andmixtures thereof.
 8. The process according to claim 1, wherein the acidformate prepared is selected from the group consisting of potassiumdiformate, sodium diformate, sodium tetraformate and mixtures thereof.9. The process according to claim 2, wherein, in the process stage (c),the removal of the methanol by distillation and the reaction of themethyl formate with the water and basic compound with transfer into thestream comprising formate and water are carried out together in onecolumn.
 10. The process according to claim 2, wherein, in the processstage (d): (i) the stream comprising the formic acid and the water fromthe process stage (b), together with the mother liquor recirculated fromstep (iv), is concentrated in a column or an evaporator with removal ofwater by distillation; (ii) the stream which was produced from step (i)by concentration and comprises formic acid, water and formate iscombined with the stream comprising the formate and water from theprocess stage (c) forming a mixture comprising the acid formate andwater; (iii) solid acid formate from the mixture comprising acid formateand water obtained from step (ii) is precipitated by crystallization andthis is isolated; and (iv) the resultant mother liquor is recirculatedto step (i).
 11. The process according to claim 3, wherein, in theprocess stage (d): (i) the stream comprising the formic acid and thewater from the process stage (b), together with the mother liquorrecirculated from step (iv), is concentrated in a column or anevaporator with removal of water by distillation; (ii) the stream whichwas produced from step (i) by concentration and comprises formic acid,water and formate is combined with the stream comprising the formate andwater from the process stage (c) forming a mixture comprising the acidformate and water; (iii) solid acid formate from the mixture comprisingacid formate and water obtained from step (ii) is precipitated bycrystallization and this is isolated; and (iv) the resultant motherliquor is recirculated to step (i).
 12. The process according to claim2, wherein, in process stage (d): (i) the stream from the process stage(b) comprising the formic acid and the water and the stream from theprocess stage (c) comprising the formate and the water are combined toform a mixture comprising the acid formate and water in a column or anevaporator with removal of water by distillation; and (ii) solid acidformate is separated off by spray granulation, spray drying or meltcrystallization from the mixture obtained from step (i) comprising acidformate and water, and this solid acid formate is isolated.
 13. Theprocess according to claim 3, wherein, in process stage (d): (i) thestream from the process stage (b) comprising the formic acid and thewater and the stream from the process stage (c) comprising the formateand the water are combined to form a mixture comprising the acid formateand water in a column or an evaporator with removal of water bydistillation; and (ii) solid acid formate is separated off by spraygranulation, spray drying or melt crystallization from the mixtureobtained from step (i) comprising acid formate and water, and this solidacid formate is isolated.